Pour water (distilled or filtered) into the spray bottle.
Add a few drops of a chosen essential oil to the water. The number of drops will depend on the size of the spray bottle. The smaller the amount of water the smaller is the number of drops you will add. Just put a few drops and mist, see what you like and add according to your preference.
Use essential oil, NOT artificial scents, as they are hormone disruptors among other things.
The type of essential oil you will chose will depend on what you will use the mist for.
You can research reputable websites that describe the properties of various essential oils.
I often recommend lavender essential oil to promote relaxation, it is soothing and calming. Mist your sheets and pillows before bed and it has a soothing and calming effect. I also mist my sheet and pillow prior to making the bed as it freshens up the sheets as it is also an antiseptic. You can also mist your face (close your eyes) and it too will have a relaxing effect. But be cautious as it may be irritating if you have sensitive skin or if you have an allergy.
It is up to you to choose other essential oils if the purpose is other than relaxation… read up on the properties of rosemary, geranium, chamomille, peppermint, grapefruit, lemon, etc..